1. Field of the Invention
The present invention relates to a semiconductor device including a circuit portion which requires light shielding, and more particularly to a semiconductor device which prevents light from entering a circuit portion to stabilize a circuit operation.
2. Related Background Art
Let us consider a photo-sensor device having a photo-sensor and a peripheral circuit such as an amplifier formed on one substrate.
FIG. 1 shows a sectional view of a related photo-sensor 1 which is basically disclosed in Japanese Patent Application Laid-Open No. 61457/1986.
A photo-sensor PD, an MOS transistor MOS and a bipolar transistor BI are formed in a P semiconductor substrate 1 with P.sup.+ regions formed therebetween.
In the photo-sensor PD, a P.sup.+ region 3 is formed in an N.sup.- region 2 to form a P.sup.+ -N.sup.- photo-diode. In the MOS transistor MOS, P.sup.+ regions 5 are formed in an N.sup.- region 4 as source and drain regions, and in the bipolar transistor BI, a P.sup.+ region 7 as a base region is formed in an N.sup.- region 6.
A gate oxidization film having a thickness of 500 .ANG. A is formed on the substrate 1 and regions in which N.sup.+ regions are to be formed are etched away. Phosphorus doped polysilcon is deposited and patterned to form an electrode 8 of the photo-diode, a gate electrode 9 of the MOS transistor and an emitter electrode 10 and a collector electrode 11 of the bipolar transistor. Then, an oxide film 12 having a thickness of 1500-2000 .ANG. is formed by thermal oxidization, and the impurity phosphorus in the polysilicon is diffused into the substrate 1 to form an N.sup.+ region 13, an emitter region 14 and an N.sup.+ region 15.
A PSG film 16 having a thickness of 6000 .ANG. is formed on the oxidization film 12 by a CVD method, contact holes are formed in the oxidization film 12 and the PSG film 16, and Al wirings 17 are formed on the elements. Then, a plasma nitride film 18 is formed, an Al light shielding layer 19 is formed, and a passivation plasma nitride film 20 is formed. Then, the plasma nitride films 20 and 18 and the light shielding layer 19 on the photo-diode PD are removed by plasma etching to form a photo-sensing area 21.
In this arrangement, external light merely impinges on the photo-diode PD through the photo-sensing area 21 and the external light is blocked in other areas by the light shielding layer 19.
Under a strong external light, block resin is coated except on the photo-sensing area 21 to form double-molding.
With such a semiconductor device, if strong light impinges on the device obliquely, light may impinge on a portion of the device which should be shielded from the light, with the result that an output characteristic of the device is influenced.
FIG. 2 shows a sectional view of an end of a prior art device. When light 22 impinges on the device obliquely, it is multi-reflected between an interface of the substrate 1 and an interface of the light shielding layer 19 and finally reaches another component in the device, which may, for example, be a transistor. Usually, light intensity rapidly attenuates by as a result of several reflections but if the light intensity is high, the light may still reach the other component after the multi-reflection, and the output characteristic of the device is affected. Such multi-reflection also occurs between interfaces formed when insulation layers of different refraction coefficients are stacked.
The output characteristic is also affected when the light obliquely impinges from the photo-sensing area 21.